The invention lies in the field of beneficiating ores.
The present invention is an improvement in the process disclosed in the above-referenced patents for recovering aluminum from alunite ore, the prior process broadly comprising dehydrating the particulate ore by roasting, removing sulfur and alkali metals compounds from the roasted ore by one of the alternatives of a reducing roast followed by an oxidation roast and leaching with solvent as disclosed in U.S. Pat. No. 3,890,425 or by an alkaline leach as disclosed in U.S. Pat. No. 3,890,426, resulting in a residue containing aluminum values and a solution containing potassium sulfate which may optionally be recovered, digesting the residue with alkaline solution to convert the aluminum values to soluble aluminate, removing silica from the aluminate solution, and precipitating the aluminum values as aluminum hydroxide from which alumina is recovered. The expression "roasting and leaching to remove water and compounds of sulfur and alkali metals" as used herein includes the above alternatives.
This invention is an improvement of the above process in which the particulate alunite ore is dehydrated by exposing the ore to burning carbonaceous material in a fluidized bed fluidized by oxygen, air, or other oxygen containing gas. The invention is particularly applicable to dehydrating ores at relatively low temperatures.
In the recovery of metals from a number of minerals by various beneficiation procedures, the removal of combined and uncombined water from the mineral is frequently required as the first step before further processing. A good many minerals of this type contain water of hydration and surface water. Commercial processes for removing water from the minerals include roasting, combusting fuels such as, liquid hydrocarbons, in the presence of the minerals in a fluidized bed, and other methods.
Roasting the ores sometimes has the disadvantage that it drives off important by-products or causes chemical reactions which preclude the recovery of valuable by-products in further processing of the ores. In the case of the dehydration of alunite ore in particular, conventional roasting techniques in the absence of sufficient oxygen result in chemical reduction of the sulfate constituents of the alunite. This is particularly disadvantageous when reduction is to be performed following dehydration since the off gases from reduction contain potentially valuable components and any reduction simultaneous with dehydration would cause loss of these useful gases.
Dehydration by combusting fuels, such as, liquid and gaseous hydrocarbons at the high temperatures required, in the presence of the minerals in a fludized bed, is subject to the disadvantage that the fuels, before combustion, generally adhere to the surfaces of the mineral particles and then burn causing localized high temperatures above the desired processing temperature range with consequent damage. High combustion temperatures promote reduction of sulfates. High temperatures also result in the reaction of alumina formed by the reduction of sulfates with silica impurities present in the mineral to form silicates which are insoluble in alkaline solutions with reduced recovery of aluminum values.
Accordingly, it is an object of this invention to provide a method for dehydrating ores at relatively low temperatures prior to further processing to recover metals therefrom, which method does not result in the generation of localized high temperatures in the ores, or in the promotion of chemical reactions which might preclude the recovery of valuable products in the subsequent processing or result in reduced recovery of metals.